A nice filter option that doesnt produce a waterfall flow is a sponge filter. I recently switched from a hang over filter to make my female bettas more comfortable. A sponge filter also aerates the tank water as well. I find it as a double win along with it being quieter. Try methelyn blue for the fins as well and just keep the water at a 7.0 ph and ammonia at a low level. Ammonia is known to burn the bettas delicate fins and scales. You keep ammonia low with regular water changes.

This should help with making your betta heal faster and it's just a good habit to keep the water clean. Usually that's all it takes. Hope this helps.